There is considerable interest in monitoring properties of semiconductors at various stages during the fabrication process. Monitoring the properties during fabrication allows the manufacturer to detect and correct technological process problems prior to the completion of the wafer.
Inspection of actual product wafers during or between technological process steps usually require non-contact techniques. Accordingly, a number of tools have been developed for optically inspecting semiconductor wafers. Such tools include reflectometers and ellipsometers. To increase the robustness of the measurements, these tools can often obtain measurements at multiple wavelengths and/or multiple angles of incidence.
Therma-Wave, Inc., the assignee of the subject invention has developed a number of such tools over the last fifteen years. One of the first such tools is described in U.S. Pat. No. 4,999,014. In this tool, a probe beam from a laser is tightly focused on the sample with a high numerical aperture lens to create light rays with a spread of angles of incidence. The reflected beam is imaged onto an array detector. The location of the elements on the array detector can be mapped to different angles of incidence on the sample. This configuration is still in commercial use today in Therma-Wave's Opti-Probe® product line and is referred to as Beam Profile Reflectometry® or BPR®.
This concept was subsequently extended to ellipsometric measurement as described in U.S. Pat. No. 5,042,951. In this approach, the change in polarization state of the probe beam is monitored at multiple angles of incidence. Various polarizers and a waveplate (or compensator) are used to permit the polarization analysis. A variant of this approach which integrates the angular information is disclosed in U.S. Pat. No. 5,181,080. This configuration is also in commercial use and is referred to as Beam Profile Ellipsometry® or BPE®.
While the latter patents were directed primarily to single wavelength systems, efforts have been made to extend these concepts to multiple wavelength systems. See for example, U.S. Pat. Nos. 5,412,473 and 5,596,411.
The assignee herein has also made efforts to improve broadband spectroscopic ellipsometry. More specifically, and as described in U.S. Pat. Nos. 5,877,859, 5,973,787; 6,134,012; 6,320,657; 6,449,043, and 6,650,415, an improved spectroscopic ellipsometer system was proposed that utilized a rotating compensator (waveplate), Prior to these disclosures, rotating compensators were typically used only in narrow band ellipsometers, while rotating polarizers were used in broadband, spectroscopic ellipsometers. The above-mentioned patents disclose how a rotating compensator can be used in BPR and BPE type systems.
In an ideal ellipsometer having a rotating element (i.e., compensator), the DC component remains constant during the entire measurement. However, in all practical applications of ellipsometers, the DC component will change over time at different angular positions of the rotating element creating noise and decreasing accuracy of ellipsometric measurements.
Signal normalization to the DC component is one of the techniques known to improve accuracy and precision in ellipsometric measurements. Examples of DC normalization in rotating element spectroscopic ellipsometer systems can be found in U.S. Pat. Nos. 6,084,675 and 6,353,477. In these patents, the DC normalization is performed for the ellipsometric signal obtained during a full revolution of the rotating element. More specifically, the DC component is computed based on a full revolution of the rotating element and the Fourier coefficients are normalized against this DC component. While this approach improves the analysis, further improvements are desirable. All of the patent cited in the background section are incorporated herein by reference.